1. Field of the Invention
The present invention relates to photosensitive paste compositions, to barrier ribs of plasma display panels (PDPs) prepared using the same, and to PDPs including the barrier ribs.
2. Description of the Related Art
In PDPs, barrier ribs are structures formed on the rear panel (or rear substrate) which define discharge spaces and prevent electrical or optical crosstalk between adjacent discharge cells. The barrier ribs may have various shapes (such as stripes or matrices) and sizes (such as width and pitch) depending on the type of PDP.
The barrier ribs are formed on the dielectric layer, which covers the address electrodes on the lower substrate of the PDP. The barrier ribs are formed by screen printing, sand blasting, etching, photolithography, or the like.
When the barrier ribs are formed by screen printing, a barrier rib paste is printed on a substrate using a patterned mask and a squeegee, and the paste is dried to remove solvent. The printing and drying steps are repeated several times to obtain a film with the desired thickness. The obtained film is sintered to form the barrier rib. However, this method is time-consuming, and presents difficulties in forming uniformly patterned barrier ribs due to misalignments caused by low resolution and repeated printing.
When the barrier ribs are formed by sand blasting, a barrier rib paste is printed once on a substrate by a table coater and then dried to obtain a film with the desired thickness. A dry film resist with sanding resistance is then laminated on the film and patterned through exposure to light and development. Then, micro-abrasive sandblasting is performed under high pressure using the dry film resist pattern as a mask to obtain a patterned film. The residual dry film resist is removed and the patterned film is sintered to complete the barrier ribs. Although barrier ribs formed by sandblasting have higher resolution than those formed by screen printing, sandblasting is a complicated process and can damage electrode terminals due to collision of the terminals with abrasive sand.
Etching is similar to sandblasting. Etching, unlike sandblasting, forms barrier ribs by patterning a sintered membrane using an etching solution. Etching can produce high resolution, but manufacturing costs are high.
Photolithography forms barrier ribs by printing and drying a photosensitive paste to form a film with the desired thickness. The film is exposed to UV light by a UV exposure system equipped with a photomask. The printing, drying, and exposure steps are repeated at least twice. The film is then developed to selectively remove unexposed regions, followed by sintering to obtain the barrier ribs. Photolithography can form barrier ribs with higher resolution than screen printing, and does not require a dry film resist or sanding process as required for sandblasting. However, photolithography requires many processes and misalignment can occur due to the repeated processes.
Therefore, methods have been developed for forming barrier ribs by exposure to light only once by minimizing the difference between the light refractive indices of inorganic and organic materials. This minimizes scattering and reflection of light irradiated at the interface between the inorganic and organic materials during exposure. Such methods are widely used.
However, these methods have several problems. First, to minimize the difference between refractive indices of the inorganic and organic materials, inorganic or organic materials having a refractive index in a specific range (which is not generally used) has to be used. The inorganic materials of the barrier rib materials that are generally used in sand blasting, etching, or the like generally have a refractive index ranging from 1.60-1.80, and the organic materials generally have a refractive index ranging from 1.50-1.60 or less. Thus, the difference between the refractive indices of these inorganic and organic materials can not be minimized. Since the inorganic materials have a high refractive index, a material having a lower refractive index has to be developed and used to minimize the difference between the refractive indices of the inorganic and organic material. On the other hand, an organic material having a higher refractive index has to be developed and used.
Second, to use a material having a high refractive index as the organic material, a binder containing Br atoms (which are harmful to the human body) has to be used. Also, an expensive monomer containing S has to be used.
Third, in sand blasting, etching or the like, a sintered powder of titania, alumina, silica, zirconia oxide, yttria, magnesia, zinc oxide, manganese oxide, iron oxide, tin oxide, copper oxide, lead oxide, or the like is used to maintain the shape of the barrier ribs, increase the reflective index of the barrier ribs, or color the barrier ribs. However, in methods requiring minimization of the difference between refractive indices of organic and inorganic materials, these powders cannot be used because the powders have very high refractive indices, and thus the difference between refractive indices of the organic material and the powder cannot be minimized. This causes the powders to interfere with the transmittance of ultraviolet rays irradiated during exposure to light so that barrier ribs can not be formed by one exposure to light.
Due to these problems, glass having a high melting point and a low refractive index and that is transparent to ultraviolet rays is used instead of the powder. As a result, the prepared barrier ribs have a low reflective index, and thus reducing the reflective index of visible rays emitted when the PDP operates, leading to reduced luminance compared with barrier ribs prepared by sand blasting or etching. To compensate for the reduced luminance, the barrier ribs are produced with narrower widths than the widths of barrier ribs prepared using other methods. This maximizes the coated phosphor area.
Lastly, barrier ribs prepared by minimizing the difference between the refractive indices of the organic and inorganic materials have higher surface roughnesses than the barrier ribs prepared using other methods. This is because when the barrier ribs are formed one exposure to light, the ultraviolet rays irradiated during the exposure to light have to reach the bottom surface. To do so, the difference between refractive indices of the inorganic and organic materials must be minimized, but also, an inorganic material having a larger particle diameter than that used in other methods (particularly etching) has to be used. In addition, the surface of the portion exposed to light for curing is removed bit by bit by developing with a developing solution, and thus the surface becomes rough. The surface with roughness tends to maintain its shape after sintering, however the shape depends on the extent of sintering. Thus, the finally prepared barrier rib cannot have a higher surface roughness than barrier ribs prepared using other methods. When the membrane of the barrier rib becomes rough, reflection efficiency with respect to visible rays is reduced, and thus crosstalk between discharge cells can be induced besides, reductions in luminance occur, the breaking strength of the panel is reduced, and noises caused by shaking of the upper and lower substrate can occur.
To increase the reflective index of the barrier ribs, titanium dioxide, alumina, zirconium oxide or silica powder has been added to the inorganic material. However, as described above, these components have high refractive indices, and although small amounts of the components are added, light exposure sensitivity is significantly reduced. Accordingly, use of the components is limited.
In addition, 5-80 nm sized particles of titanium dioxide, alumina, silica, zirconium oxide, yttria, or the like has been used. Alternatively, the reflective index of the barrier ribs has been increased by adding 5-80 nm sized particles of titanium dioxide, alumina, silica, zirconium oxide, yttria, magnesia, zinc oxide, manganese oxide, iron oxide, tin oxide, copper oxide or lead oxide. This imparts some improvement, since the particles having very small diameters compared to the wavelength of light irradiated during exposure do not cause scattering, reflection or the like with respect to light, and cause an increase in the refractive index of the organic material. Accordingly, there is no need to use an inorganic material having a low refractive index and an organic material having a high refractive index. However, when the barrier ribs are prepared by simply adding the 5-80 nm sized particles by mixing, 3-roll milling or the like, the particles are not dispersed in the paste in a single particle state and are instead agglomerated with one another.
In addition, a method of increasing the luminance of the barrier ribs by adding a metal sol prepared in the presence of polyhydric alcohol has been proposed. However, the paste is prepared by minimizing only the difference between refractive indices of the inorganic and organic material, not taking into consideration the refractive index and specific gravity of the metal sol. Therefore, an inorganic material having a low refractive index and an organic material having a high refractive index have to be used.